Flexible wood structures and devices, and methods for fabricating and use thereof

ABSTRACT

A flexible structure is formed by subjecting cellulose-based natural wood material to a chemical treatment that partially removes hemicellulose and lignin therefrom. The treated wood has a unique 3-D porous structure with numerous channels, excellent biodegradability and biocompatibility, and improved flexibility as compared to the natural wood. By further modifying the treated wood, the structure can be adapted to particular applications. For example, nanoparticles, nanowires, carbon nanotubes, or any other coating or material can be added to the treated wood to form a hybrid structure. In some embodiments, open lumina within the structure can be at least partially filled with a non-wood substance, such as a flexible polymer, or with entangled cellulose nanofibers. The unique architecture and superior properties of the flexible wood allow for its use in various applications, such as, but not limited to, structural materials, solar thermal devices, flexible electronics, tissue engineering, thermal management, and energy storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 62/480,909, filed Apr. 3, 2017, which is herebyincorporated by reference herein in its entirety.

FIELD

The present disclosure relates generally to natural wood structures, andmore particularly, to chemical treatment of natural wood to haveincreased flexibility, as well as structures and devices incorporatingsuch flexible wood.

SUMMARY

Embodiments of the disclosed subject matter provide a flexible structureformed by subjecting cellulose-based natural wood material to a chemicaltreatment that partially removes hemicellulose and lignin therefrom. Thetreated wood has a unique three-dimensional (3-D) porous structure withnumerous channels, excellent biodegradability and biocompatibility, andimproved flexibility as compared to the natural wood. By furthermodifying the treated wood, the structure can be adapted to particularapplications. For example, nanoparticles, nanowires, carbon nanotubes,or any other coating or material can be added to the treated wood toform a hybrid structure. In some embodiments, open lumina within thestructure can be at least partially filled with a non-wood substance,such as a flexible polymer, or with entangled cellulose nanofibers. Theunique architecture and superior properties of the flexible wood allowfor its use in various applications, such as, but not limited to,structural materials, solar thermal evaporation devices, flexibleelectronics, tissue engineering, and energy storage.

In one or more embodiments, a structure comprises a flexible substrateof natural wood that has been chemically modified to partially removehemicellulose and lignin therein while substantially preserving astructure of cellulose-based lumina.

In one or more embodiments, a method comprises treating a piece ofnatural wood with a chemical solution so as to partially removehemicellulose and lignin therein while substantially preserving astructure of cellulose-based lumina, and then drying the piece ofchemically-treated natural wood. The treating and the drying increasethe flexibility of the piece.

In one or more embodiments, a structure is formed by treating naturalwood with a chemical solution that partially removes hemicellulose andlignin therein while substantially preserving a structure ofcellulose-based lumina, and the treated piece of natural wood has abending radius that is at least 2 times smaller than that of the naturalwood before treatment.

In one or more embodiments, a hybrid structure comprises a flexiblesubstrate and at least one of nanoparticles, nanowires, graphene,graphite, single-walled carbon nanotubes, double-walled carbonnanotubes, and multi-walled carbon nanotubes coupled to a surfacethereof, where the flexible substrate comprises natural wood that hasbeen chemically modified to partially remove hemicellulose and lignintherein while substantially preserving a structure of cellulose-basedlumina.

In one or more embodiments, a hybrid structure comprises a flexiblesubstrate and a non-wood material. The flexible substrate comprisesnatural wood that has been chemically modified to partially removehemicellulose and lignin therein while substantially preserving astructure of cellulose-based lumina. The non-wood material is coupled tosurfaces forming said lumina.

Objects and advantages of embodiments of the disclosed subject matterwill become apparent from the following description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will hereinafter be described with reference to theaccompanying drawings, which have not necessarily been drawn to scale.Where applicable, some elements may be simplified or otherwise notillustrated in order to assist in the illustration and description ofunderlying features. Throughout the figures, like reference numeralsdenote like elements.

FIG. 1A is a simplified illustration of the structure of a piece ofnatural wood.

FIG. 1B is a simplified top down view of fiber cells in a natural woodstructure.

FIG. 1C is a scanning electron microscope (SEM) image of thecross-section of the natural wood structure showing individual fibercells.

FIG. 1D is an SEM image of a longitudinal section of the natural woodstructure showing elongated lumina.

FIG. 1E are images of natural wood subjected to a bending force thatleads to fracture.

FIG. 2A is a simplified illustration of the structure of a piece offlexible wood, according to one or more embodiments of the disclosedsubject matter.

FIG. 2B is a simplified top down view of at least partiallycrumpled/collapsed fiber cells in the flexible wood structure, accordingto one or more embodiments of the disclosed subject matter.

FIG. 2C is an SEM image of the cross-section of the flexible woodstructure showing numerous crumpled cells, according to one or moreembodiments of the disclosed subject matter.

FIG. 2D is an SEM image of a longitudinal section of the flexible woodstructure showing narrowed elongated lumina, according to one or moreembodiments of the disclosed subject matter.

FIG. 2E are images of flexible wood subjected to a bending force withoutfracture, according to one or more embodiments of the disclosed subjectmatter.

FIG. 2F is a magnified SEM image showing the aligned cellulosenanofibers in the flexible wood structure, according to one or moreembodiments of the disclosed subject matter.

FIG. 2G is an image showing flexible wood wrapped around a fingerwithout fracture, according to one or more embodiments of the disclosedsubject matter.

FIG. 2H is an image showing flexible wood subjected to extreme twistingwithout fracture, according to one or more embodiments of the disclosedsubject matter.

FIG. 2I is an illustration of flexible wood wrapped around an appendage(wrist) for use in wearable applications, according to one or moreembodiments of the disclosed subject matter.

FIG. 2J is an image showing flexible wood wrapped around an appendagewithout fracture, according to one or more embodiments of the disclosedsubject matter.

FIG. 3 is a process flow diagram for fabricating flexible-wood-basedstructures, according to one or more embodiments of the disclosedsubject matter.

FIG. 4A is a graph of stress-strain curves by tensile test for naturalwood structure and the flexible wood structure of FIG. 2A.

FIG. 4B is an illustration of the force application for the tensiletests underlying the stress-strain curves of FIG. 4A.

FIG. 5A is a simplified schematic of the flexible wood structure of FIG.2A subjected to bending.

FIG. 5B is an SEM image of a cross-section of the flexible woodstructure in bending, according to one or more embodiments of thedisclosed subject matter.

FIG. 5C is a magnified SEM image of region 506 in FIG. 5B, illustratingaligned cellulose nanofibers, according to one or more embodiments ofthe disclosed subject matter.

FIG. 5D is an SEM image from a top view (502) of the flexible woodstructure in bending, according to one or more embodiments of thedisclosed subject matter.

FIG. 5E is an SEM image from a bottom view (504) of the flexible woodstructure in bending, according to one or more embodiments of thedisclosed subject matter.

FIG. 5F is an SEM image from the top view (502) of the flexible woodstructure after release from bending, according to one or moreembodiments of the disclosed subject matter.

FIG. 5G is an SEM image from the bottom view (504) of the flexible woodstructure after release from bending, according to one or moreembodiments of the disclosed subject matter.

FIG. 6A is a simplified schematic diagram of a cross-section of aflexible wood structure as a biological scaffold, according to one ormore embodiments of the disclosed subject matter.

FIG. 6B is a simplified isometric illustration of a flexible woodstructure as a biological scaffold, according to one or more embodimentsof the disclosed subject matter.

FIG. 6C is an SEM image of HEK293 cells cultured on a surface of theflexible wood biological scaffold, according to one or more embodimentsof the disclosed subject matter.

FIG. 6D is a magnified SEM image of region 660 in FIG. 6C.

FIG. 6E is an SEM image of HEK293 cells cultured on surfaces withinlumina of the flexible wood biological scaffold, according to one ormore embodiments of the disclosed subject matter.

FIG. 6F is a magnified SEM image of region 680 in FIG. 6E.

FIG. 7 is a simplified process flow for modification 312 of FIG. 3 forforming a flexible wood biological scaffold, according to one or moreembodiments of the disclosed subject matter.

FIG. 8A is a simplified schematic diagram of a cross-section of aflexible wood structure as a vapor generation device, according to oneor more embodiments of the disclosed subject matter.

FIG. 8B is a simplified cutaway illustration of an individual cell inthe flexible wood structure as vapor generation device, according to oneor more embodiments of the disclosed subject matter.

FIG. 8C is a simplified isometric illustration of the flexible woodstructure as vapor generation device, according to one or moreembodiments of the disclosed subject matter.

FIG. 8D is a magnified view illustrating a simplified structure ofcarbon-nanotube coating on microsheet surfaces of the flexible woodstructure as vapor generation device, according to one or moreembodiments of the disclosed subject matter.

FIG. 8E is a simplified schematic diagram of a solar thermaldistillation device including a flexible wood vapor generation device,according to one or more embodiments of the disclosed subject matter.

FIG. 8F is a graph of steam generation performance of a flexible woodvapor generation device under different solar concentrations, accordingto one or more embodiments of the disclosed subject matter.

FIG. 8G is a graph of cycling performance of the flexible wood vaporgeneration device under a solar concentration of 7 sun, according to oneor more embodiments of the disclosed subject matter.

FIG. 9 is a simplified process flow for modification 312 of FIG. 3 forforming a flexible wood vapor generation device, according to one ormore embodiments of the disclosed subject matter.

FIG. 10A is a simplified schematic diagram of a cross-section of aflexible wood structure as a battery, according to one or moreembodiments of the disclosed subject matter.

FIG. 10B is a simplified cutaway illustration of an individual cell inthe flexible wood structure as battery, according to one or moreembodiments of the disclosed subject matter.

FIG. 10C is a close-up sectional illustration of an individual cellshowing separate transport pathways for the flexible wood structure as aLi—O₂ battery, according to one or more embodiments of the disclosedsubject matter.

FIG. 10D is an SEM image of a top surface of the flexible wood structureas battery, showing flower-like microsheets, according to one or moreembodiments of the disclosed subject matter.

FIG. 10E is an SEM image of a cross-section and top surface of theflexible wood structure as battery, showing multi-channeled structurewith big vessels and small lumina, according to one or more embodimentsof the disclosed subject matter.

FIG. 10F is a magnified SEM image of a surface of the flexible woodstructure as battery, showing carbon-nanotube coating and nanoparticles,according to one or more embodiments of the disclosed subject matter.

FIG. 10G is a transmission electron microscope (TEM) image of a surfaceof the flexible wood structure as battery, showing carbon-nanotubecoating and nanoparticles, according to one or more embodiments of thedisclosed subject matter.

FIG. 11A is a graph illustrating charge-discharge profiles at 200 mA/gfor various cycles of the flexible wood structure as Li—O₂ battery,according to one or more embodiments of the disclosed subject matter.

FIG. 11B is a graph illustrating average charge and dischargedpotentials over an initial 100 cycles at 200 mA/g for the flexible woodstructure as Li—O₂ battery, according to one or more embodiments of thedisclosed subject matter.

FIG. 11C is a graph illustrating cycling performance of the flexiblewood structure as Li—O₂ battery as compared to other structures, for afixed capacity of 1000 mAh/g and a stable Coulombic efficiency of ˜100%,according to one or more embodiments of the disclosed subject matter.

FIG. 11D is an SEM image of the top surface of the flexible woodstructure as Li—O₂ battery after 100 cycles of charge/discharge,according to one or more embodiments of the disclosed subject matter.

FIG. 11E is an SEM image of the cross-section of the flexible woodstructure as Li—O₂ battery after 100 cycles of charge/discharge,according to one or more embodiments of the disclosed subject matter.

FIG. 12A is a simplified illustration of discharge-charge reactions of asingle cell of the flexible wood structure as Li—O₂ battery, accordingto one or more embodiments of the disclosed subject matter.

FIG. 12B is a simplified illustration of the reversible electrochemicalreactions occurring at the tri-phase interface of the flexible woodstructure as Li—O₂ battery, according to one or more embodiments of thedisclosed subject matter.

FIG. 13 is a simplified process flow for modification 312 of FIG. 3 forforming a flexible wood battery, according to one or more embodiments ofthe disclosed subject matter.

FIG. 14A is a simplified schematic diagram of a cross-section of aflexible wood structure with direction of extension of lumina parallelto a top surface thereof, according to one or more embodiments of thedisclosed subject matter.

FIG. 14B is a simplified schematic of the flexible wood structure ofFIG. 14A subjected to bending.

FIG. 15 is a simplified schematic diagram of a cross-section of aflexible wood structure with microsheets on multiple surfaces, accordingto one or more embodiments of the disclosed subject matter.

FIG. 16 is a simplified schematic diagram of a cross-section of a hybridflexible wood structure with nanoparticles intercalated into cell wall202, according to one or more embodiments of the disclosed subjectmatter.

FIG. 17 is a simplified schematic diagram of a cross-section of anotherhybrid flexible wood structure including conductive elements, accordingto one or more embodiments of the disclosed subject matter

FIG. 18 is a simplified schematic diagram of a cross-section of anotherhybrid flexible wood structure with filled lumina, according to one ormore embodiments of the disclosed subject matter.

FIG. 19A is a simplified illustration of a flexible wood with entangledcellulose nanofibers acting as a hydrogel, according to one or moreembodiments of the disclosed subject matter.

FIG. 19B is a simplified illustration of a close-up cross-sectional viewof the lumina of the flexible wood structure of FIG. 19A, showingentangled cellulose fibers, according to one or more embodiments of thedisclosed subject matter.

FIG. 19C is an SEM image of the cross-section of the flexible woodstructure showing entangled nanofibers within the lumina, according toone or more embodiments of the disclosed subject matter.

FIG. 20 is a process flow diagram for fabricating flexible woodstructures with entangled nanofibers, according to one or moreembodiments of the disclosed subject matter.

FIG. 21A is a graph of stress-strain curves by compression test fornatural wood structure (wet and dry) and the flexible wood structure ofFIG. 19A.

FIG. 21B is an illustration of the force application for the compressiontests underlying the stress-strain curves of FIG. 21A.

DETAILED DESCRIPTION

Natural wood materials have a lignocellulosic composition that allow forexcellent biodegradability and biocompatibility. Natural wood also has aunique three-dimensional porous structure with multiple channels,including tracheids and vessel cells in the xylem, with the cell typesdepending on the species of the wood. The cell walls of the natural woodare mainly composed of cellulose, hemicellulose, and lignin. The threecomponents intertwine with each other forming a strong and rigid cellwall structure. Cellulose is the crystalline fibril with long, slenderchains of glucose. Hemicellulose is a type of amorphousheteropolysaccharide. Lignin is a polyphenol-based material filling inthe spaces between cellulose and hemicellulose, and acts as anadhesive-like agent in the cell wall structure.

In embodiments of the disclosed subject matter, natural wood issubjected to a chemical treatment in order to partially remove ligninand hemicellulose therefrom, while substantially retaining theunderlying cellulose-based structure. The wood-based material resultingfrom the chemical treatment can enjoy at least improved flexibility ascompared to the natural wood prior to the treatment. Moreover,additional materials can be added to the wood-based material after thechemical treatment in order to form hybrid structures. The addedmaterials can add functionality not otherwise available with the naturalwood, for example, by providing conductivity or thermal absorptivity,while enjoying the improved mechanical performance offered by thewood-based material post chemical treatment.

As used herein, flexible wood refers to natural wood that has beensubjected to the chemical treatment processes described herein, whichprocesses partially remove lignin and hemicellulose and therebyadvantageously alter the mechanical properties of the wood. The termssubstrate and membrane are used interchangeably herein and refer to aparticular piece, strip, block, membrane, or any other shape of wood.Indeed, in some instances, the flexible wood substrate or flexible woodmembrane is simply referred to as flexible substrate or flexiblemembrane. In some embodiments, the substrate or membrane may beconsidered thin, i.e., having a thickness less than either dimension ina plane perpendicular to the thickness direction.

Referring initially to FIG. 3, a generalized process 300 for forming andusing flexible wood substrates is shown. The process 300 can optionallybegin at 302, where a piece of natural wood is supplied, for example, bycutting from an existing tree or block of natural wood. For example,FIG. 1A illustrates a piece of natural wood 100 that has been cut to adesired rectangular shape, although other shapes are also possible,while FIG. 1B illustrates a subsection of the lumina 104 of the naturalwood 100. The natural wood 100 exhibits a unique three-dimensionalstructure with irregular hexagonal lumina 104 of 30-50 μm in diameteralong the tree-growth direction. The lumina 104 are defined by cellwalls 102, which are primarily composed of cellulose. FIGS. 1C and 1Dare scanning electron microscopy (SEM) images showing the morphology andmicrostructure of natural wood 100, in this case balsa wood.

The natural wood can be any type of hard wood or softwood, such as, butnot limited to, basswood, oak, poplar, ash, alder, aspen, balsa wood,beech, birch, cherry, butternut, chestnut, cocobolo, elm, hickory,maple, oak, padauk, plum, walnut, willow, yellow poplar, bald cypress,cedar, cypress, douglas fir, fir, hemlock, larch, pine, redwood, spruce,tamarack, juniper and yew.

In some embodiments, cutting 302 can include modifying a geometry orprofile of one or more surfaces of the natural wood prior to chemicaltreatment. For example, the cutting 302 can be such that a roughenedsurface is formed on the natural wood. Such roughened surface may be aflower-like rough surface with flower-like microsheets extendingtherefrom, for example, as described below with respect to FIGS. 8A-8D,10A-10G, and 15.

After the cutting 302, the process 300 proceeds to 304 where the pieceof natural wood 100 is subjected to treatment with a chemical solutionto partially (but not fully) remove lignin and hemicellulose therefrom.The treatment is such that between 5% and 95% of the lignin in theoriginal natural wood has been removed, while retaining at least some ofthe cellulose of the natural wood. For example, at least 20%, at least40%, at least 60%, or at least 90% of the cellulose from the naturalwood is retained after the chemical treatment. The piece of natural woodmay be immersed in the chemical solution and subject to vacuum, so thatthe chemical solution can better infiltrate the structure and channelsof the natural wood.

The chemical solution can include at least one of NaOH, Na₂SO₃,(NH₄)₂SO₃, p-TsOH, NH₃.H₂O, NaOH+Na₂S, Na₂CO₃, NaOH+Na₂SO₃,NaOH+(NH₄)₂SO₃, NH₄OH+(NH₄)₂SO₃, NH₄OH+Na₂SO₃, NaOH+Na₂CO₃, NaOH+AQ,NaOH/Na₂S+AQ, NaOH+Na₂SO₃+AQ, Na₂SO₃+AQ, NaOH+Na₂S+Na₂S_(n),Na₂SO₃+NaOH+CH₃OH+AQ, C₂H₅OH+NaOH, C₂H₈N₂, C₂H₇NO+NH₃—H₂O, N₂H₄—H₂O, andNaHCO₃, where n in an integer and AQ is Anthraquinone. However, themechanical properties of the treated wood substrate may depend on the pHof the chemical solution used for the treatment. In particular,solutions having a pH value greater than 7 should be used in order toproduce a treated wood substrate with improved flexibility.

For example, when natural wood is treated with water (i.e., pH=7) or HCl(i.e., pH<7) solutions, the mechanical flexibilities of the resultingsubstrates are improved slightly but still remain relatively rigid. As aresult, the substrates are still subject to breakage during bending. Incontrast, substrates treated with a mixed solution of NaOH and Na₂SO₃(i.e., pH>7) show substantially improved mechanical flexibility, withthe ability to undergo severe folding and twisting (e.g., at least twosimultaneous folds, folding onto itself, and/or twists greater than 45°)without breakage and to revert to its original shape upon removal of thedeforming force.

While not wishing to be bound by any particular theory, the unique pHresponse may be attributed to the distinct stabilities of cellulose,hemicellulose, and lignin in solutions of different pH value. Forexample, NaOH can be used to selectively degrade lignin andhemicellulose in the wood materials while having little effect on thecellulose components. The addition of Na₂SO₃ can help further remove thelignin component and reduce the reaction time by increasing sulphitegroups (SO₃ ²⁻) in the lignin side chains via sulphonation, therebyallowing the lignin to more readily dissolve in alkaline solution. Theinteraction between the NaOH/Na₂SO₃ and cellulose, hemicellulose, andlignin molecules can also swell the cell wall, making it softer forsubsequent manipulations (e.g., bending, folding, or twisting).Accordingly, in some embodiments, an aqueous solution of 2.5M NaOH and0.4M of Na₂SO₃ mixed together can be used for as the chemical solutionfor the treatment of 304, although other solution compositions selectedfrom the above list or otherwise that satisfy the pH requirement arealso contemplated for 304.

At 306, it is determined if sufficient lignin has been removed from thenatural wood. As noted above, between 5% and 95% of the lignin isremoved, with the ultimate amount being dependent on the desiredmechanical properties of the treated substrate for a particularapplication. The amount of lignin removed is dependent on the immersiontime, which may range from as little as 10 minutes to over 7 hours, forexample, 1 hour. Depending on the desired amount of lignin removal, thetemperature of the chemical solution may also be adjusted in addition toor in place of adjusting the time of immersion. For example, in someembodiments, the chemical solution may be boiling, which may effect agreater amount of lignin removal (e.g., for a given immersion time),whereas non-boiling solutions may be used for removing small amounts oflignin.

Once sufficient lignin has been removed by the treatment of 304, theprocess 300 can proceed to optionally rinsing the treated piece of woodat 308. The rinsing can include immersing the treated piece of wood in asolvent, such as, but not limited to, de-ionized (DI) water. In someembodiments, the solvent may be at an elevated temperature, such asboiling. The rinsing 308 can be effective to remove any remnants of thechemical solution within the treated piece of wood and/or any componentsof the wood dislodged by the treatment 304. In some embodiments, it maybe desirable to retain the dislodged wood components, in which case therinsing 308 can be omitted, for example, as described below with respectto FIG. 20.

After rinsing 308 (or after conclusion of treatment 304 when there is norinsing 308), the process 300 can proceed to 310, wherein the treatedpiece of wood is subjected to drying. Depending on the desiredapplication for the treated wood, it may be subject to air drying (andthe attendant crumpling of lumina) or drying that substantially retainsan open structure of the lumina (e.g., freeze drying or critical pointdrying). For example, in applications where mechanical strength may bemore desirable, the air drying may result in improved flexibility andhigher strength due to the crumpled and shrunken lumina. For otherapplications, it may be desirable to preserve the original open latticehexagonal lumina structure, for example, for transport of speciesthrough the substrate 200. In such cases, freeze drying or criticalpoint drying may be used.

For example, FIG. 2A illustrates a piece of treated wood 200 that hasbeen subject to air drying, and FIG. 2B illustrates a subsection oflumina 204 with crumpled cell walls 202 of the treated wood 200. Afterthe chemical treatment 304, the multi-channeled three-dimensional porousstructure of the natural wood 100 is preserved in the treated wood 200.However, the structure 200 has evolved from an open latticed hexagonallumina 104 to crumpled lumina 204 with irregular shape and reducedcross-sectional dimension. The drying leads to the shrinkage of the cellwalls 202 with closely packed cellulose nanofibers, resulting in thecrumpled lumina 204. FIGS. 2C and 2D are scanning electron microscopy(SEM) images showing the morphology and microstructure of the treatedwood 200.

Significant shrinkage can be observed during the drying process,primarily due to dehydration of the structure. For example, the flexiblewood substrate 200 can have a weight of at least 5% less, 10% less, orabout 20% less (e.g., 20.7%) than that of the natural wood 100 beforethe chemical treatment, and dimensions W₂, t₂, of the flexible woodsubstrate can decrease by at least 5%, 20%, or about 35% from respectivedimensions W₁, t₁, of the natural wood 100 before the chemicaltreatment.

The crumpled, shrunken lumen structure can contribute to the improvedmechanical properties of the chemically-treated wood substrate 200. Inparticular, while not wishing to be bound by any theory, the crumpledcell walls can sustain both tensile and compression forces due to theflexibility of cellulose nanofibers and extensive inter-fiber hydrogenbonds. FIG. 4A shows experimental stress-strain curves by tensile tests(with force application as illustrated in FIG. 4B) for natural wood 100and the flexible wood substrate 200. Mechanical properties of naturalwood 100 and the flexible wood substrate 200 are further reflected inTable 1 below.

As is apparent from FIG. 4A and Table 1, the mechanical tensile strengthof the flexible substrate 200 is substantially improved over that of thenatural wood 100 by the process 300. For example, the mechanical tensilestrength of the flexible wood substrate can increase by at least 2times, at least 5 times, or at least 7 times as compared to themechanical tensile strength of the original natural wood.

TABLE 1 Comparison of mechanical properties for natural wood (balsawood) and chemically treated wood Strength Modulus Toughness Bend Radius(MPa) (MPa) (MJ/m³) (mm) Natural Wood (100) 0.49 134 0.001 5.1  TreatedWood (200) 3.29 116 0.10  0.06 Change 6.7x 0.86x 100x 85x IncreaseDecrease Increase Decrease

As shown in FIG. 1E, the natural wood substrate 100 readily breaks uponbending. In contrast, one can subject the flexible wood substrate 200 toextreme bending—i.e., folding upon itself (with at least twosimultaneous folds)—without breaking, as shown in FIG. 2E. Upon releaseof the deforming force, the flexible wood substrate 200 can return toits original state, e.g., a flat shape, without substantial defect ordamage. Indeed, a relatively thick (e.g., 4 mm) flexible wood substrate200 can be subjected to severe twisting—i.e., twists of at least 45°, oreven multiple twists of at least 90°—without breaking, as shown in FIG.2H, and can return to its original state after release.

After partially removing the hemicellulose and lignin by chemicaltreatment, the cellulose nanofibers forming walls 202 remain aligned, asshows in FIG. 2F, but nanopores can be introduced between the cellulosenanofibers, thereby increasing the softness and flexibility of thechannel walls 202. For example, the modulus of the flexible woodsubstrate 200 can decrease by at least 10%, such as from 134 MPa to 116MPa, as compared to the modulus of the original natural wood 100.Moreover, the alignment of cellulose nanofibers may enable the flexiblewood substrate 200 to have anisotropic properties.

The superior flexible nature of the flexible wood substrate 200 allowsthe substrate to be adapted to a variety of different shapes andconfigurations. For example, the substrate 200 can be deformed to beworn by a user for a wearable device. In such a configuration, thesubstrate 200 can be wrapped around an appendage of a user, for example,a finger 206 as shown in FIG. 2G or a wrist 208 as shown in FIGS. 2I-2J,or any other body part.

Moreover, the flexible wood shows excellent durability after undergoing1000 bending cycles, suggesting outstanding structural stability inbending. For example, the bend radius of the flexible wood substrate 200can decrease by at least 2 times, at least 10 times, or at least 50times as compared to the bending radius of the original natural wood100. For example, the bend radius of the natural wood substrate 100 canbe 5.1 mm (i.e., curvature of 0.195 mm⁻¹) while the bend radius of theflexible wood substrate 200 can be 0.06 mm (i.e., curvature of 16.7mm⁻¹), as shown in Table 1. Unlike conventional wood structures thathave grooves cut into their surface to increase their flexibility, theflexible wood substrate can be considered a monolithic piece, withsubstantially continuous flat top and bottom surfaces.

As noted above, the unique crumpled lumina may be beneficial to theflexibility of the flexible wood substrate 200. When subjected to abending force as illustrated in FIG. 5A, the top side 502 of theflexible wood substrate 200 will sustain tension force while the bottomside 504 sustains compression force. As a result, the top side 502 ofthe lumina 204 will expand whereas the bottom side 504 of the luminashrinks, resulting in multiple cone-shaped channels. FIG. 5D shows a topview SEM image of the bent flexible wood substrate 200, where the upperparts of the lumina are more open than in their original state. Thecrumpled cell walls in the unbent flexible wood substrate 200 arepartially straightened due to the tension induced by the bending, atleast partially returning to a hexagonal cell shape similar to thenatural wood 100 prior to chemical treatment.

In contrast, FIG. 5E shows a bottom view SEM image of the bent flexiblewood substrate 200, where the bottom parts of the lumina are heavilycompressed so as to be almost closed and with more severe curving ofcell walls. Yet, the cellulose nanofibers are maintained in alignment,with smaller gaps due to the compression of the bottom part, as shown inFIGS. 5B-5C. When released, the flexible wood can fully recover itsoriginal shape without any noticeable damage to the underlyingstructure. As illustrated in FIGS. 5F and 5G, respectively, the top andbottom part of the lumina revert to their original crumpled state,indicative of the structural recovery capability of the flexible woodsubstrate 200.

Returning to FIG. 3, after the drying 310, the flexible wood substrate200 can optionally be modified for use in a particular application at312. Thus, 312 can include adding materials to the flexible woodsubstrate 200, for example, by coupling to exterior or interior surfacesthereof, in order to add functionality that the flexible wood substrate200 (or the original natural wood 100) otherwise lacks, while stillbenefiting from the improved mechanical characteristics offered by thetreatment 304. The addition of materials to the flexible wood substrate200 can in effect form a hybrid structure of wood and non-native (e.g.,non-wood or wood cells of a different species) materials specificallyadapted to a particular application.

For example, modification 312 can include applying a coating to externalsurfaces and/or internal surfaces of the flexible wood substrate 200, orcoupling particles to the external surfaces and/or internal surfaces ofthe flexible wood substrate 200. The coating, or the coupled particles,can include a conductive material, a semiconductive material, or aninsulating material. For example, the coating, or the coupled particles,can include at least one of nanoparticles, nanowires, graphene,graphite, single-walled carbon nanotubes (CNTs), double-walled CNTs,multi-walled CNTs, polyaniline, and carbon black.

In some embodiments, the coating, or the coupled particles, can be asolar or radiation absorptive material, such as, but not limited to,CNTs, carbon black, graphite, hard carbon, reduced graphene oxide, orgraphene. In some embodiments, the coating, or the coupled particles,can include plasmonic metallic nanoparticles, catalytic nanoparticles,or electroactive nanoparticles. Examples of materials for the plasmonicmetallic nanoparticles include but are not limited to Au, Pt, Ag, Pd,and Ru.

In some embodiments, the coating, or the coupled particles, can includeat least one of metallic nanoparticles, metal alloy nanoparticles,semiconductor nanoparticles, sulfides, phosphides, borides, and oxides.The metallic nanoparticles and the metal alloy nanoparticles caninclude, but are not limited to, Pt, Pd, Au, Ag, Ni, Co, Ru, and Fe.Examples of materials for the semiconductor nanoparticles includeCuFeSe₂ or any other semiconductor. Examples of materials for thesulfides include, but are not limited to, MoS₂, CoS_(x), and FeS₂, wherex is an integer. Examples of materials for the phosphides include, butare not limited to, CoP, NiP₂, and MoP_(x), where x is an integer.Examples of materials for the borides include, but are not limited to,CoB, MoB, and NiB. Examples of materials for the oxides include, but arenot limited to, MnO₂, Fe₂O₃, CoO, and NiO.

As such, the flexible wood substrate 200 can be adapted at 312 for awide variety of uses at 314. Examples of such uses 314 include, but arenot limited to, building (construction) or structural materials,biological scaffolds for tissue engineering, biocompatible/biodegradableor implantable structures, steam or vapor generation devices ordistillation systems, energy storage devices, flexible conductors,flexible electronic devices, wearable devices, and shape memorystructures. While the following discussion details adaptation of theflexible wood substrate 200 for specific use examples, embodiments ofthe disclosed subject matter are not limited thereto. Indeed, one ofordinary skill in the art will appreciate that the flexible woodsubstrate 200 can be readily adapted to other uses beyond thoseexplicitly discussed herein through application of the teachings of thepresent disclosure.

As noted above, the flexible wood substrate 200 has a uniquethree-dimensional porous structure with numerous channels and excellentbreathability, biodegradability and biocompatibility by virtue of thecellulose-based structure of the natural wood 100 precursor, whileenjoying increased flexibility as a result of the partial lignin andhemicellulose removal. In addition, the multiple direct micro-channelsand nano-channels in the substrate can be beneficial to nutrienttransportation and metabolic byproduct removal. These propertiescontribute to biocompatibility and efficient cell adhesion of non-woodcells (e.g., human or animal cells) that allow the flexible woodsubstrate to act as three-dimensional cell carriers or biologicalscaffold for tissue engineering. Thus, in one embodiment, the flexiblewood substrate serves a biological scaffold.

FIG. 6A shows an embodiment of a biological scaffold 600 incorporatingthe flexible wood substrate 200. One or more cells 602 can adhere to anexternal surface of the substrate 200. Additionally or alternatively,one or more cells 604 can adhere to internal surfaces of the substrate200, i.e., the cellulose-based cell walls 202 defining the lumina 204.In generally, the cells 602, 604 are non-wood cells, such as human oranimal cells. For example, successful attachment of human embryonickidney (HEK) 293 cells have been demonstrated on a flexible woodsubstrate 200, as illustrated in FIGS. 6B-6F.

In particular, FIGS. 6C and 6D are SEM images of a top of a flexiblewood substrate 200, showing the formation of HEK293 cells 602 on thesurface thereof, with FIG. 6D being a magnified view of region 660 inFIG. 6C. FIGS. 6E and 6F are SEM images of a longitudinal section of theflexible wood substrate 200, showing infiltration and attachment of theHEK293 cells 604 within the lumina 204 of the substrate 200, with FIG.6F being a magnified view of region 680 of FIG. 6E. Thus, efficientthree-dimensional adhesion and proliferation of non-wood cells in theflexible wood substrate can be realized, which is a necessary step inapplications for three-dimensional bio-scaffolding and tissueengineering.

The culturing of the non-wood cells in the flexible wood substrate doesnot substantially affect the beneficial mechanical properties of thesubstrate. Thus, the flexibility of the flexible wood substrate ismaintained. Repeated bending of the substrate does not affect adherenceof the HEK293 thereto, indicating a highly stable and sufficientattachment property.

FIG. 7 shows a simplified process flow for the modification 312 of FIG.3 when the flexible wood substrate 200 is used as a biological scaffold.For example, modification 312 can include, at 702, seeding a pluralityof the non-wood cells on the flexible wood substrate 200. After theseeding 702, the modification 312 can proceed to culturing the non-woodcells at 704. Culturing 704 can include transporting nutrients to thenon-wood cells and metabolic byproducts from the non-wood cells via thepreserved cellulose-based structures of the flexible wood substrate 200,e.g., the partially collapsed lumina 204 and/or nanochannels betweencellulose nanofibers within cell walls 202. The culturing 704 may form acell population, cell culture, or at least part of a tissue. After 704,the biological scaffold 600 may then be subject to use at 314 in FIG. 3.Such use can include, but is not limited to, further culturing orimplanting of the formed population or tissue into a patient, which maybe a human or animal.

In another embodiment, a coating can be added to at least one surface ofthe flexible wood substrate 200 in order to form a hybrid structure.FIG. 8A illustrates an embodiment of hybrid device 800, where a topsurface region 802 of flexible wood substrate 200 has been provided witha coating 806. In some embodiments, the top surface region 802 may befurther modified to include a particular surface geometry, such asprotrusions or microsheets 804, as illustrated in FIG. 8A. Depending onthe application, the coating 806 can be a radiation absorbing coating, aradiation reflective coating, an electrically or thermally conductivecoating, a semiconductive coating, or an electrically or thermallyinsulating coating.

For example, the hybrid device 800 can be configured as a vaporgeneration device, as illustrated in FIGS. 8B-8E. As described above,the natural wood structure is subjected to treatment with a chemicalsolution to partially remove lignin and hemicellulose, thereby resultingin a flexible wood substrate 200 suitable for portable applications suchas the vapor generation device. The resulting composition of anexemplary flexible wood substrate 200 is reflected below in Table 2. Asnoted above, the chemically treated substrate can be subject to freezedrying (e.g., for 2 days) or critical point drying in order to maintainan unblocked structure for the lumina.

TABLE 2 Composition of natural wood and resulting treated wood for usein vapor generation Cellulose Hemicellulose Lignin Natural Wood (100)40.0% 15.9% 24.0% Treated Wood (200) 37.6% 11.6% 19.8%

The flexible wood substrate 200 can be further modified to optimize itfor vapor generation. For example, prior to the chemical treatment, thenatural wood 100 can be cut to create a rough flower-like surface 802with numerous flower-like microsheets 804, as illustrated in FIGS.8A-8B. For example, the cut can be performed with an electric saw. Themicrosheets 804 increase the surface area and elongate the optical pathfor multiple scattering, thereby enhancing the light absorbability. Inaddition, at least the rough surface with the microsheets 804 is coatedwith a light absorbing coating 806 to improve the photothermalconversion efficiency.

In the example discussed below, the light absorbing coating 806comprises CNTs. However, embodiments of the disclosed subject matter arenot limited thereto and other materials can be used for the lightabsorbing coating 806. Moreover, in the example discussed below, thefluid is water and the resulting vapor is steam. However, embodiments ofthe disclosed subject matter are not limited thereto and other fluidsare also possible according to one or more contemplated embodiments.Similarly, although solar illumination is discussed as the radiationsource in the example below, other radiation sources are also possibleaccording to one or more contemplated embodiments.

In a fabricated example, the mass percentage of the coating 806 of theCNTs was as low as 0.3-0.5 wt %. The flower-like surface was uniformlycoated with a layer 806 of CNTs, while the inside channels 204 remainedsmooth without CNT coating. Strong interactions and connections betweenCNTs and the wood matrix of the substrate 200 result from the abundant—OH and —COOH groups on the surface of the CNTs and —OH groups on thecellulose, as illustrated in FIG. 8D. The CNT coating 806 of theflexible wood substrate 200 does not substantially affect the beneficialmechanical properties of the substrate 200. Thus, the flexibility of theflexible wood substrate 200 is maintained. Repeated bending of thesubstrate 200 does not affect adherence of the coating 806 thereto,indicating a highly stable and sufficient attachment property.

In the fabricated example, the carbon nanotube (CNT) coated black andflower-like surface 802 of the flexible wood substrate 200 with largesurface area can maximize the light absorption by elongating the opticalpath for multiple scattering and thereby promote photothermalconversion. FIGS. 8C-8D illustrate an example of a vapor generationdevice 800, where FIG. 8D is a magnified view of region 815 in FIG. 8C,illustrating the coupling between the CNT coating and the surface of theflexible wood substrate 200.

Thermal losses can be minimized by localizing the photothermalgeneration at the water-air interface 812 with the thermally insulatingwood matrix 202, in particular by restricting the coating 806 to anupper region of the device 800. The chemically treated wood itself isalso a good thermal insulator, such that the majority of the generatedheat will be localized at the top surface 802 of the substrate 200. Dueto its relatively low density, the device 800 can float on the water,such that interface 812 can be formed between the water and thesubstrate 200 closer to the top surface 802 than the bottom, e.g., nearthe CNT coating 806.

As illustrated in FIG. 8C, when the sunlight 808 illuminates the vaporgeneration device 800, the rough coated flower-like surface 802 willabsorb the sunlight 808 and generate localized heating at the fluid-airinterface 812. As a result, the local temperature increases andevaporates the fluid. The resulting vapor 810 escapes to the atmosphere,or can be captured for distillation purposes (e.g., converting saltwater to freshwater, or converting contaminated water to potable water).

For example, as illustrated in FIG. 8E, a distillation device 820including the vapor generation device 800 can use a capture hood 822 tocapture the generated vapor 810. The capture hood 822 may be constructedto allow solar radiation 808 to pass therethrough to the vaporgeneration device 800. Optionally, the capture hood 822 can includefocusing optics to increase an intensity of the radiation 808 incidenton the absorbing surface 802 of the vapor generation device 800. Thecaptured vapor can be directed to a condenser 824 to convert the vaporback to a fluid, which can then be stored in a container 826 for lateror immediate use.

Returning to FIGS. 8B-8C, as the water proximal to the upper surface 802continuously evaporates, water from the bottom of the flexible woodsubstrate 200 will simultaneously be pumped through the substrate 200,via vessels and other natural interconnected channels in the wood suchas connected lumen 204. In particular, the negative pressure at the topof the substrate 200 induces capillary forces within the substratechannels that have smaller diameters than vessels, thereby moving fluidto the top 802 of the substrate 200.

Treatment of the natural wood with the chemical solution can break sometracheid cell walls to form large microchannels through connectingadjacent cell lumen 204, both vertically and tangentially, forpotentially transpiring water. There also exist nanopores (i.e.,nanochannels) between the aligned cellulose fibrils of walls 202 due tothe partial removal of hemicellulose and lignin, which can improve thewater transpiration capability of the wood matrix through capillarity.These characteristics of the flexible wood substrate 200 ensure acontinuous supply of water for evaporation, without requiring a separatepumping mechanism. Thus, fluid can be passively pumped to the top heatedsurface 802 from the opposing bottom surface, which may be disposed onor in a body of the fluid, via capillary action and/or nano-cavitationeffects.

The CNT-coated 806 flexible wood substrate 200 with a flower-likesurface and multichannels along the tree-growth direction was evaluatedusing a solar simulator as a light source, the vapor generator 800 as alight absorber, and a beaker with water as fluid source. Uponirradiation, the temperature of the surface of the substrate 200 rapidlyincreased due to the local heat generation. Steam is thus generated andincreases as the radiation intensity increases. The resultingevaporation rate (E.R.) as a function of time is shown in FIG. 8F. As isapparent, each curve shows an evaporation rate that increases rapidlywithin the initial 5 minutes, and then slowly reaches a maximum value.By virtue of the above noted optimization of light absorption, thermalmanagement, fluid transportation, and evaporation, a steam generationdevice 800 based on the flexible wood substrate 200 can demonstrate aremarkably high efficiency (e.g., at least 60% at 10 kW/m² or at least80% at 10 kW/m²). As shown in FIG. 8G, the steam generation deviceemploying the flexible wood substrate exhibits stable evaporation ratesfor 20 cycles.

FIG. 9 shows a simplified process flow for the modification 312 of FIG.3 when the flexible wood substrate 200 is used in a vapor generationdevice 800. For example, modification 312 can include, at 902, coatingat least the microsheet surface 804 of the vapor generation device 800.When the coating comprises CNTs, a CNT powder can be dispersed in asolvent such as acetone to make a CNT solution. The flexible woodsubstrate 200, or at least an upper surface 804 thereof, can be immersedinto the CNT solution. During the immersion, the CNT solution and/or thesubstrate 200 may optionally be sonicated. Factors affecting the coatinginclude the thickness of the substrate 200 and immersion time. If thethickness of the substrate is relatively large (as when the substrate200 is used in the vapor generation device 800) the infiltration of theCNTs into the channels 204 is comparatively more difficult. Moreover,the immersion and/or sonication time may be relatively short to furtherinhibit CNT penetration into the channels 204.

The substrate 200 can then be removed from the solution and allowed toair dry 904 in order to build a layer of CNTs. The coating 902 anddrying 904 can be repeated at 906 to build the coating layer by layer onthe surface of the flexible substrate 200. One of ordinary skill in theart will appreciate that other coating processes for modification 312,as applied to a vapor generation device or any other deviceincorporating the flexible wood substrate, would be applicable todifferent coating materials and are within the scope of the disclosedsubject matter.

In another embodiment, a coating can be added to at least a top surfaceand interior surfaces of the cell walls 202 that form the lumina 204 ofthe flexible wood structure in order to form another hybrid structure.FIG. 10A illustrates an embodiment of hybrid device 1000, where both atop surface region 1008 and surfaces of walls 202 of the flexible woodsubstrate 200 have been provided with a coating 1004. In someembodiments, the top surface region 1008 may be further modified toinclude a particular surface geometry, such as protrusions ormicrosheets 804, as illustrated in FIG. 10A. Depending on theapplication, the coating 1004 can be a radiation absorbing coating, aradiation reflective coating, an electrically or thermally conductivecoating, a semiconductive coating, or an electrically or thermallyinsulating coating.

Alternatively or additionally, one or more non-wood particles 1006 canbe coupled to surfaces of the flexible substrate 200, either directly tothe wood surface or through coating 1006 when available. For example,depending on the application, the particles 1006 can be catalyticnanoparticles, electroactive nanoparticles, plasmonic metallicnanoparticles, semiconductor nanoparticles, metallic or metal alloynanoparticles, sulfides, phosphides, borides, or oxides, among othermaterials.

For example, the hybrid device 1000 can be configured as an energystorage device, i.e., a battery cell, as illustrated in FIGS. 10B-12B.As described above, the natural wood structure is subjected to treatmentwith a chemical solution to partially remove lignin and hemicellulose,thereby resulting in a flexible wood substrate 200 suitable for portableapplications such as the battery cell. As also noted above, thechemically treated substrate can be subject to freeze drying (e.g., for2 days) or critical point drying in order to maintain an open structurefor the lumina 204.

When configured as a battery cell 1000, the hybrid device can furtherinclude a separator membrane 1010 that separates an anode 1012 from theflexible substrate 200, which acts as a cathode 1002 and currentcollector. The flexible substrate 200 can have surfaces coated by anelectrically conductive coating 1004. A housing 1018 may enclose theanode 1012, cathode 1002, and separator membrane 1010. An appropriateelectrolyte fluid may fill the nanochannels of cell wall 202 withoutobstructing the open spaces of lumina 204. In a charging state,electrons may flow in direction 1016 from cathode 1002 through powersupply/load 1019 to the anode 1012, whereas electrons may flow in theopposite direction 1014 in a discharging state.

In the example discussed below, the electrically conductive coating 1004comprises CNTs. However, embodiments of the disclosed subject matter arenot limited thereto and other materials can be used for the electricallyconductive coating 806. Moreover, in the example discussed below, thebattery is configured as a Li—O₂ battery, with the anode 1012 being a Lifoil. However, embodiments of the disclosed subject matter are notlimited thereto and other battery cell compositions are also possibleaccording to one or more contemplated embodiments. Similarly, althoughRu nanoparticles are discussed as the particles 1006 coupled to theflexible substrate 200 via coating 1004, other particles (whethermacro-, micro-, or nano-particles) are also possible according to one ormore contemplated embodiments.

For example, coating 1004 on the top surface and within channels 204 ofthe flexible substrate 200 can include CNTs, which may couple to thesurface of the cell walls 202 by electrostatic interaction between thecellulose and the CNTs. As illustrated in FIGS. 10D and 10E,respectively, both the flower-like rough surface 1008 with numerousmicrosheets and the multi-channeled structure (e.g., including biggervessels and smaller lumina 204) can be preserved through the chemicaltreatment to partially remove lignin and hemicellulose and thesubsequent coating with CNTs.

In a battery cell embodiment 1000 employing the flexible wood substrate200, the battery operation takes advantage of noncompetitive andcontinuous pathways offered by the structure of the coated flexiblesubstrate for transport of ions, gas, and electrons. For example, thecellulose nanofiber cell wall 202 has abundant nanopores 1021 betweenthe nanofibers 1020, which can be used to transport ions 1026 (e.g., Li⁺ions) when soaked with moderate amount of electrolyte, as illustrated inFIG. 10C. The wood lumina 204 and vessels (i.e., channels) can act asthe continuous transport pathway for gas 1024 (e.g., O₂), as illustratedin FIG. 10B. With a small amount of catalytic particles 1006 (e.g.,ruthenium (Ru)) loaded onto the surface of the coating 1004, thesubstrate 200 can serve as both cathode and current collector. Thecatalytic particles 1006 on the surface of the coating 1004 can bebeneficial to the redox reaction involving oxygen reduction andevolution, which can work with the other benefits of the flexiblesubstrate 200 to achieve a high electrochemical performance with bothhigh capacity and long cycling life.

For example, the battery cell 1000 is formed with the modified flexiblesubstrate 200 as a current collector-free cathode, a glass fibermembrane as separator 1010, and a lithium foil as anode 1012. Moderateamount of liquid electrolyte (e.g., 1M LiTFSI/TEGDME electrolyte) can beadded to the battery cell to ensure that sufficient electrolytepermeated the cellulose nanofiber walls 202 of the substrate 200, so asto form a continuous pathway for ion transport, but without extraelectrolyte that would otherwise flood the channels 204. The openchannels 204 also provide sufficient space for growth of Li₂O₂ productwithout substantially obstructing the flow of oxygen gas in the channels204. The rough flower-like surface 1008 and abundant pores with uniformloading of catalytic nanoparticles 1004 provides plenty of active sitesfor redox reactions.

Moreover, the flexible substrate 200 maintains its excellent mechanicalproperties, allowing the resulting battery 1000 to be manipulated ordeformed without negatively impacting performance or reliability. Forexample, battery cells incorporating the modified flexible substrate canbe subjected to bending, folding, rolling, twisting, etc. withoutapparent degradation in performance or stability and can revert to theiroriginal shape after manipulation. For example, the battery cell can befolded for 1440° or rolled completely upon itself without degradation inperformance. Accordingly, the battery incorporating the flexiblesubstrate 200 may be particularly useful in applications for wearableand portable electronics.

Due to the underlying wood structure and the design of the battery cell,the modified flexible wood substrate 200 demonstrates a high capacity ofover 7000 mAh/g, a long cycling life of 100 cycles with a restrictedcapacity of 1000 mAh/g at 200 mA/g, and superior electrochemical andmechanical stability. FIG. 11A shows the galvanostatic discharge-chargevoltage profiles with a restricted capacity of 1000 mAh/g at a currentdensity of 200 mA/g, for selected cycles of 1^(st), 10^(th), 30^(th),60^(th), and 100^(th) (although only 1^(st), 60^(th), and 100^(th) arelabeled for clarity). The battery cell 1000 demonstrates a relativelylow overpotential of 1.12V in the first cycle, which remains relativelystable for nearly 60 cycles, as reflected in FIG. 11B. After 60 cycles,there is a slight increase in overpotential, eventually reaching 1.58Vat 100 cycles. While not wishing to be bound by any theory, the increaseof overpotential may be due to the decomposition of electrolyte duringthe repeated charge/discharge processes.

The excellent electrochemical performance of the modified flexiblesubstrate can be attributed to the noncompetitive and continuoustri-pathway structure. FIGS. 12A-12B graphically illustrate thisbreathable tri-phase redox reaction process involving electrons, Li⁺ions, and O₂ gas, and their transport in the modified flexiblesubstrate. The reversible electrochemical reactions occurring in thetri-phase interface involve noncompetitive and continuous tri-pathways:(1) Li⁺ ions 1026 are transported along a first pathway formed by theelectrolyte-filled cell walls 202 (i.e., underneath the CNT coating 1004with Ru nanoparticles 1006); (2) electrons 1022 are transported along asecond pathway formed by the conductive CNT coating 1004 on the topsurface and surfaces of the cell walls 202; and (3) O₂ gas 1024 isallowed to flow along a third pathway formed by the open, unblockedchannels 204 of the substrate 200. When the three components meet at theinterface, reaction takes place under the influence of an externallyapplied electric field, thereby forming Li₂O₂ products 1028 during thedischarge. When charging, a reversible process occurs, along with thedisappearance of the accumulated Li₂O₂ products 1028.

Control experiments were compared to the battery cell 1000 with modifiedflexible substrate 200. In the first control experiment, the channelsand rough surface of the modified flexible substrate were eliminated bydensification (e.g., pressing) before being assembled into a Li—O₂battery cell. As shown in FIG. 11C, the densified structure(illustration and curve on the left) delivers a very limited capacity ofapproximately 250 mAh/g at a current density of 200 mA/g and with a muchlarger overpotential. The elimination of the rough surface and channelsalso significantly reduces the active sites for redox reactions, therebydecreasing the capacity greatly. The now blocked channels in thedensified substrate prevents the continuous supply of O₂ gas, thusrequiring the battery cell to rely on dissolved gas in the electrolyte.This further reduces the capacity of the battery cell. In addition, theLi₂O₂ product will continuously accumulate on the flat surface of theelectrode, forming an insulating layer that can enhance theoverpotential significantly.

In the second control experiment (middle illustration and curve in FIG.11C), the same electrode structure as the modified flexible substratewas used, but with overweight electrolyte to flood the channels therein.As shown in FIG. 11C, the flooded structure delivers only half capacityof the unflooded counterpart (illustration and curve on the right) witha slightly increased overpotential. The significant reduction incapacity can be attributed to the blocking of channels by the extraelectrolyte and thus preventing the continuous transport of O₂ gas fromthe outside to the cathode site. The slight reduction in overpotentialcan be attributed to the well-preserved rough surface and channels, bothof which provide space for the reversible growth of the Li₂O₂ product.

A battery cell incorporating the modified flexible substrate alsoexhibits excellent rechargeability. For example, the battery cell canreversibly discharge for 100 cycles with a restricted capacity of 1000mAh/g at a current density of 200 mA/g. This excellent rechargeabilityis due to the stable structures of the modified flexible support. Asillustrated in FIGS. 11D-11E, the multi-channeled structure with roughflower-like surface 1008 of the modified flexible substrate ismaintained even after 100 cycles of discharging and charging. Bothcoated CNTs 1004 and Ru nanoparticles 1006 are observed, confirming thatthere is a strong adhesion between the coating 1004 and the wood matrixof the cell walls 202.

As noted above, during the discharging process a layer of reactant 1028(e.g., Li₂O₂) coats the top surface and inside surface of the channels204, as illustrated in FIGS. 12A-12B. However, the layer of the reactant1028 is not sufficiently thick to otherwise totally block the channels204, thereby allowing O₂ gas 1024 to be efficiently transported throughthe channels 204 during the charging/discharging process. Suchbreathable character is attractive for metal-air battery designs. At theend of the charging process, the reactant 1028 almost completelydisappears from the substrate, thereby exposing the CNT coating 1004 andRu 1006 nanoparticles on the top surface of the substrate 200 and innersurfaces of the channels 204 therein.

FIG. 13 shows a simplified process flow for the modification 312 of FIG.3 when the flexible wood substrate 200 is used in a battery cell 1000.For example, modification 312 can include, at 1302, coating at least thetop surface (e.g., surface 1008 with microsheets) and inner surfaces(e.g., cell walls 202 defining the lumina 204) of the flexible substrate200. When the coating 1004 comprises CNTs, CNT powder can be dispersedin a solvent such as acetone to make a CNT solution. The flexible woodsubstrate 200 can be immersed into the CNT solution. During theimmersion, the CNT solution and/or the substrate 200 may optionally besonicated.

As noted above, factors affecting the coating include the thickness ofthe substrate 200 and immersion time. If the thickness of the substrate200 is relatively thin (as in the battery cell embodiments) theinfiltration of the CNTs into the channels 204 is comparatively easier.The sonication treating time may also be relatively longer to furtherencourage CNT penetration into the channels 204. Moreover, most of anyblockages in the channels 204 are broken down by the prior treatmentwith chemical solution for partial lignin removal, thereby resulting insubstantially unblocked channels, as shown in FIG. 10D. The CNT solutioncan thus easily penetrate the channels.

The substrate 200 can then be removed from the solution and allowed toair dry 1304 in order to build a layer of CNTs. The coating 1302 anddrying 1304 can be repeated at 1306 to build the coating 1004 layer bylayer on the surfaces of the flexible substrate 200. One of ordinaryskill in the art will appreciate that other coating processes formodification 312, as applied to an energy storage device or any otherdevice incorporating the flexible wood substrate 200, would beapplicable to different coating materials and are within the scope ofthe disclosed subject matter. A uniform coating 1004 is thus disposed onthe top surface and inner surfaces of the channels, thereby forming aninterconnected and continuous network for electron transport.

Modification 312 can further include deposition of non-wood particles1006, such as catalytic nanoparticles (or any other particle), at 1308.The deposition of non-wood particles can occur after the formation ofthe coating 1004, as illustrated in FIG. 13, before forming the coating1004, or in between formation of different layers of the coating 1004.

For example, the deposited non-wood particles 1006 can be Runanoparticles, which can be coated onto the surface of the CNT coating1004 to act as a catalyst for the oxygen reduction and evolutionreactions. In such a configuration, at 1310, the flexible wood substrate200 can be soaked in a solution of ruthenium chloride (RuCl₃) (e.g.,concentration of 12 mg/ml) for 12 hours. After 1310, at 1312, theflexible wood substrate 200 can be soaked in a solution of sodiumborohydride (NaBH₄) in order to reduce the Ru ions.

As illustrated in FIG. 10F, numerous tiny Ru nanoparticles 1006 areformed on the surface of the CNT coating 1004. Further confirmation ofRu nanoparticle formation is provided by the transmission electronmicroscopy (TEM) image of FIG. 10G. Clear lattices with a spacing of0.21 nm can be observed, thus confirming the crystal structure of the Runanoparticles. The particle size of the Ru nanoparticles is very small,with an average diameter of 2.3 nm.

For use 314 of the flexible wood substrate 200 in a battery cellconfiguration, the modified substrate resulting from 1312 can then beassembled together with a separator membrane 1010 and anode 1012, asdescribed above. Moreover, the cell walls 202 of the substrate 200 canbe filled with an electrolyte, as also described above.

Although many of the examples discussed herein have a direction ofextension of the lumina 204 perpendicular to a top surface (i.e.,parallel to a thickness direction of the substrate 200), embodiments ofthe disclosed subject matter are not limited thereto. Indeed, it ispossible to have the direction 1406 of extension of the lumina 204 beparallel to a top surface 1402 or a bottom surface 1404 (i.e.,perpendicular to a thickness direction of the substrate), as illustratedfor the substrate 1400 of FIGS. 14A-14B. Of course, other orientationsfor the lumina 204 are also possible according to one or morecontemplated embodiments. For example, the direction of extension of thelumina 204 can be at an arbitrary angle within the substrate (i.e.,neither aligned with nor perpendicular to a surface of the substrate orto a thickness direction of the substrate).

Moreover, although FIGS. 8A-8D and 10A-12B employ microsheets 804 on onesurface of the substrate (e.g., the top surface), embodiments of thedisclosed subject matter are not limited thereto. Rather, more than onesurface may also have microsheets 804 according to one or morecontemplated embodiments of the disclosed subject matter. For example,FIG. 15 illustrates a configuration 1500 where both a top surface 1502and a bottom surface 1504 include microsheet projections 804. AlthoughFIG. 15 illustrates a coating 804 on both the top surface 1502 and thebottom surface 1504, it is also possible for only one of the top surface1502 and the bottom surface 1504 to include the coating, or for allsurfaces, including the top surface 1502, the bottom surface 1504, andinner surfaces forming lumina 204 to be coated.

In addition to providing non-wood particles on one or more surfaces ofthe substrate 200, it also possible to provide particles within the woodstructure itself. For example, non-wood particles (or other non-nativeparticles) can be intercalated within the cell walls, in particular, inthe nano-sized gaps or pores between the cellulose nanofibers formingthe cell walls. FIG. 16 illustrates such a configuration 1600, whereparticles 1602 (e.g., nanoparticles) are disposed within walls 202 ofthe flexible wood substrate.

In one or more embodiments, the flexible wood substrate 200 can serve asa flexible three-dimensional electrical conductor, for example, bycombining with a conductive coating or filling to form a conductivehybrid structure. For example, by using a simple CNT coating (or anyother conductive agent, such as, but not limited to, graphene,polyaniline, or carbon black) on inner or outer surfaces of the wood,the flexible wood substrate 200 can be modified as a flexiblethree-dimensional conductor.

Moreover, the flexible wood substrate 200 may be used as a template forforming an electrical circuit with conductive traces. For example, FIG.17 illustrates a configuration 1700 where a conductive coating 1702 isprovided over inner surfaces of cell walls 202. An insulating filling1708 may optionally be disposed with the lumina 204. Conductive traces1704 disposed on external surfaces of the substrate may connect theconductive coatings 1702 within lumina 204 to form an electricalcircuit. Alternatively, the configuration 1700 may employ insulatingcoatings 1702 surrounding a semiconductive material 1708 within lumina204. Again, conductive traces 1704 disposed on external surfaces of thesubstrate may connect the semiconductive materials 1708 or otherconductive connections to form an electrical circuit.

In one or more embodiments, the open (or partially collapsed) lumina 204of the flexible wood substrate 200 can be filled with a flexiblepolymer. For example, the lumina 204 can be filled with a hydrogel or asilicone polymer. FIG. 18 illustrates such a configuration 1800 whereinlumen 204 are filled by a flexible polymer 1802, such that the substrate200 can retain its flexible nature.

In one or more embodiments, the fabrication process for the flexiblewood substrate 200 can be slightly modified to form a network ofentangled cellulose nanofibers within the lumina 204. In such aconfiguration, the substrate 200 may exhibit highly elastic, anisotropicproperties and may be considered a hydrogel. FIGS. 19A-19C illustratesuch a flexible wood substrate acting as a hydrogel 1900, wherenanofibers 1904 within each lumina 204 become entangled.

The hydrogel 1900 can be fabricated by slightly modifying thegeneralized fabrication of FIG. 3, in particular by eliminating therinsing step between the drying 310 and the chemical treatment 304, asillustrated in FIG. 20. Moreover, the drying 310 can includefreeze-drying rather than air drying or critical point drying. Since thewood substrate is not subject to rinsing, cellulose nanofibers 1904released from the cells walls 202 by the chemical treatment remainwithin the substrate rather than being washed away. These freenanofibers 1904 can reassemble and entangle with each other to form aninterconnected network within the lumina 204, as illustrated in FIG.19C. In particular, in the freeze-drying process 310 following thechemical treatment 304, any ice formation can act as template for theformation of the interconnected fiber network inside the lumina 204, thefiber network being comprised of entangled cellulose nanofibers 1904. Insome embodiments, after the drying, the hydrogel 1900 can be hydratedsuch that the inner volumes of the cellulose-based lumina can be filledwith the interconnected fiber network and a fluid (e.g., water) trappedby the fiber network.

This fiber network can imbue the hydrogel 1900 with a superior abilityto handle compression forces as well as the ability to handle tensionforces resulting from the partial lignin and hemicellulose removal.Thus, the hydrogel 1900 not only exhibits superior mechanicalflexibility in tension, similar to those properties discussed above forflexible wood substrate 200, but also superior properties incompression. For example, the hydrogel 1900 can be subjected tocompression of at least 40% (for example up to 70%) and can totallyrecover its original shape after release of the compression force. Suchsuperior performance of the hydrogel 1900 is demonstrated by FIG. 21A,which shows stress-strain curves of dry natural wood, wet natural wood,and the elastic wood hydrogel 1900 at a compression strain of 60% (withforce application as illustrated in FIG. 21B).

In addition, after the mechanical compression testing as shown in FIG.21A, the elastic wood hydrogel 1900 may experience only a negligiblechange in height (i.e., in a direction parallel to the direction offorce application), whereas the wet wood and the dry wood bothexperience significant reductions in height. For example, the heightloss of the elastic wood hydrogel may be less than 1% (e.g.,approximately 0%), while the height losses for the wet wood and the drywood both are about 32% and 44%, respectively.

As is readily apparent from the above description, many variations withthe flexible wood substrate 200 serving as a base are possible. Indeed,different substrates 200 with different modifications can be assembledtogether to yield a composite hybrid device with favorable mechanicalproperties. For example, a first flexible substrate 200 can be formed asa battery cell, a second flexible substrate 200 can be formed as aflexible three-dimensional conductor, and a third flexible substrate 200can be formed as part of an electrical circuit, with the flexibleconductor providing power from the battery cell to the electricalcircuit. The different substrates 200 may be coupled together on acommon larger flexible substrate 200 acting as a support, for example,as a wearable device. Other configurations are also possible and withinthe contemplated scope of the present disclosure.

In one or more first embodiments, a structure comprises a flexiblesubstrate of natural wood that has been chemically modified to partiallyremove hemicellulose and lignin therein while substantially preserving astructure of cellulose-based lumina.

In the first embodiments, or any other embodiment, the natural woodretains at least some of the hemicellulose and lignin after the chemicalmodification. In the first embodiments, or any other embodiment, between5% and 95% of the lignin has been removed by the chemical modification.

In the first embodiments, or any other embodiment, the natural woodretains at least 20% of the cellulose after the chemical modification.In the first embodiments, or any other embodiment, the natural woodretains at least 40% of the cellulose after the chemical modification.In the first embodiments, or any other embodiment, the natural woodretains at least 60% of the cellulose after the chemical modification.In the first embodiments, or any other embodiment, the natural woodretains at least 90% of the cellulose after the chemical modification.

In the first embodiments, or any other embodiment, in cross-sectionalview, the cellulose-based lumina have crumpled or have a shrunkendiameter as compared to the natural wood before the chemicalmodification.

In the first embodiments, or any other embodiment, the flexiblesubstrate has an increased flexibility as compared to the natural woodbefore the chemical modification. In the first embodiments, or any otherembodiment, the bending radius of the chemically modified natural woodof the flexible substrate is at least 2 times smaller than that of thenatural wood before the chemical modification. In the first embodiments,or any other embodiment, the bending radius of the chemically modifiednatural wood of the flexible substrate is at least 5 times smaller thanthat of the natural wood before the chemical modification. In the firstembodiments, or any other embodiment, the bending radius of thechemically modified natural wood of the flexible substrate is at least10 times smaller than that of the natural wood before the chemicalmodification. In the first embodiments, or any other embodiment, thebending radius of the chemically modified natural wood of the flexiblesubstrate is at least 50 times smaller than that of the natural woodbefore the chemical modification.

In the first embodiments or any other embodiment, the tensile strengthof the chemically modified natural wood of the flexible substrate is atleast 10% greater than the natural wood before the chemicalmodification. In the first embodiments or any other embodiment, thetensile strength of the chemically modified natural wood of the flexiblesubstrate is at least 2 times greater than the natural wood before thechemical modification. In the first embodiments or any other embodiment,the tensile strength of the chemically modified natural wood of theflexible substrate is at least 5 times greater than the natural woodbefore the chemical modification. In the first embodiments or any otherembodiment, the tensile strength of the chemically modified natural woodof the flexible substrate is at least 7 times greater than the naturalwood before the chemical modification.

In the first embodiments or any other embodiment, the weight of thechemically modified natural wood of the flexible substrate is at least5% less than the natural wood before the chemical modification. In thefirst embodiments or any other embodiment, the weight of the chemicallymodified natural wood of the flexible substrate is at least 20% lessthan the natural wood before the chemical modification.

In the first embodiments or any other embodiment, a dimension (e.g.,width, thickness, or length) of the chemically modified natural wood ofthe flexible substrate is at least 5% less than the natural wood beforethe chemical modification. In the first embodiments or any otherembodiment, a dimension (e.g., width, thickness, or length) of thechemically modified natural wood of the flexible substrate is at least15% less than the natural wood before the chemical modification. In thefirst embodiments or any other embodiment, a dimension (e.g., width,thickness, or length) of the chemically modified natural wood of theflexible substrate is at least 35% less than the natural wood before thechemical modification.

In the first embodiments or any other embodiment, a modulus of thechemically modified natural wood of the flexible substrate is at least1% less than the natural wood before the chemical modification. In thefirst embodiments or any other embodiment, a modulus of the chemicallymodified natural wood of the flexible substrate is at least 5% less thanthe natural wood before the chemical modification. In the firstembodiments or any other embodiment, a modulus of the chemicallymodified natural wood of the flexible substrate is at least 10% lessthan the natural wood before the chemical modification.

In the first embodiments or any other embodiment, the lumina extendperpendicular to a thickness direction of the flexible substrate. In thefirst embodiments or any other embodiment, a dimension of the flexiblesubstrate in a direction perpendicular to the thickness direction isgreater than a dimension of the flexible substrate in the thicknessdirection.

In the first embodiments or any other embodiment, the flexible substratehas sufficient flexibility so as to return to its original shape afterbeing folded onto itself or being twisted more than 45°. In the firstembodiments or any other embodiment, the flexible substrate hassufficient flexibility so as to return to its original shape after beingfolded onto itself or being twisted more than 90°. In the firstembodiments or any other embodiment, the flexible substrate hassufficient flexibility so as to return to its original shape after beingfolded onto itself or being twisted more than 135°.

In the first embodiments or any other embodiment, the natural wood ofthe flexible substrate is a monolithic piece. In the first embodimentsor any other embodiment, the flexible substrate consists essentially ofthe chemically modified natural wood.

In the first embodiments or any other embodiment, the lumina extend in athickness direction of the flexible substrate. In the first embodimentsor any other embodiment, a dimension of the flexible substrate in adirection perpendicular to the thickness direction is greater than adimension of the flexible substrate in the thickness direction.

In the first embodiments or any other embodiment, cellulose nanofibersof the chemically modified natural wood are substantially aligned alonga common direction. In the first embodiments or any other embodiment,the chemically modified natural wood has nanopores between the alignedcellulose nanofibers. In the first embodiments or any other embodiment,inner volumes of at least some of the cellulose-based lumina of theflexible substrate are open or unobstructed.

In the first embodiments or any other embodiment, inner volumes of thecellulose-based lumina are filled with an interconnected fiber networkand a fluid. In the first embodiments or any other embodiment, cellulosenanofibers within each inner volume of the cellulose-based lumina areentangled with each other. In the first embodiments or any otherembodiment, the flexible substrate is constructed to return to itsoriginal shape after being compressed by at least 20%.

In the first embodiments or any other embodiment, the natural wood ishard wood or softwood. In the first embodiments or any other embodiment,the natural wood is one of basswood, oak, poplar, ash, alder, aspen,balsa wood, beech, birch, cherry, butternut, chestnut, cocobolo, elm,hickory, maple, oak, padauk, plum, walnut, willow, yellow poplar,baldcypress, cedar, cypress, douglas fir, fir, hemlock, larch, pine,redwood, spruce, tamarack, juniper and yew.

In the first embodiments or any other embodiment, the structure furthercomprises a plurality of non-wood cells attached to cell wall surfacesforming the lumina in the flexible substrate. The flexible substrate canact as a tissue scaffold for the non-wood cells. In the firstembodiments or any other embodiment, the plurality of non-wood cellsforms a tissue on the flexible substrate. In the first embodiments orany other embodiment, the plurality of non-wood cells comprise human oranimal cells.

In the first embodiments or any other embodiment, the structure has acoating on the flexible substrate. In the first embodiments or any otherembodiment, the coating comprises a conductor, a semiconductor, or aninsulator. In the first embodiments or any other embodiment, the coatingcomprises at least one of nanoparticles, nanowires, graphene, graphite,single-walled carbon nanotubes, double-walled carbon nanotubes, andmulti-walled carbon nanotubes.

In the first embodiments or any other embodiment, the coating is alight-absorbing coating. In the first embodiments or any otherembodiment, the flexible substrate has a first surface with a pluralityof micro-sheet protrusions extending therefrom, and the light-absorbingcoating is disposed on the first surface including the micro-sheetprotrusions. In the first embodiments or any other embodiment, the firstsurface is perpendicular to a direction in which the lumina extend. Inthe first embodiments or any other embodiment, inner surfaces of thelumina away from the first surface remain uncoated by thelight-absorbing coating. In the first embodiments or any otherembodiment, the flexible substrate is constructed such that fluid ispumped to the first surface from a second surface opposite the firstsurface via capillary action and/or nano-cavitation effects. In thefirst embodiments or any other embodiment, the flexible substrate isconstructed to float on the fluid, with the second surface on or withinthe fluid. In the first embodiments or any other embodiment, the firstsurface of the flexible substrate is an evaporative surface and thestructure acts as a steam or vapor generation device.

In the first embodiments or any other embodiment, the structure acts asa steam generation device having a solar thermal efficiency of at least60% at 10 kW/m². In the first embodiments or any other embodiment, thestructure acts as a steam generation device having a solar thermalefficiency of at least 70% at 10 kW/m². In the first embodiments or anyother embodiment, the structure acts as a steam generation device havinga solar thermal efficiency of at least 80% at 10 kW/m².

In the first embodiments or any other embodiment, the coating comprisesat least one of carbon nanotubes (CNT), carbon black, graphite, hardcarbon, reduced graphene oxide, graphene, plasmonic metallicnanoparticles, and semiconductor nanoparticles. In the first embodimentsor any other embodiment, the plasmonic nanoparticles comprise at leastone of Au, Pt, Ag, Pd, and Ru. In the first embodiments or any otherembodiment, the semiconductor nanoparticles comprise CuFeSe₂ or anyother type of semiconductor. In the first embodiments or any otherembodiment, the coating on the flexible substrate is anelectrically-conductive coating.

In the first embodiments or any other embodiment, the flexible substratehas a first surface with a plurality of micro-sheet protrusionsextending therefrom, and the electrically-conductive coating is disposedon the first surface including the micro-sheet protrusions and on innersurfaces forming the lumina within the flexible substrate. In the firstembodiments or any other embodiment, the first surface is perpendicularto a direction in which the lumina extend. In the first embodiments orany other embodiment, the structure acts as at least one of a conductor,a part of an electrical circuit, and a battery.

In the first embodiments or any other embodiment, the flexible substratedefines a first pathway through cell walls of the lumina for iontransport, a second pathway through inner volumes of the lumina fortransport of a gas, and a third pathway via the electrically-conductivecoating for transport of electrons.

In the first embodiments or any other embodiment, the structurecomprises an anode, an electrolyte solution filling at least theflexible substrate, and a separator membrane disposed between theflexible substrate and the anode. The flexible substrate can act as acurrent collector-free cathode and the structure can be configured as abattery. In the first embodiments or any other embodiment, the ionstransported by the first pathway are lithium ions (Li⁺), the gastransported by the second pathways is oxygen gas (O₂), and thewood-based structure is configured as a Li—O₂ battery.

In the first embodiments or any other embodiment, the structure has aplurality of catalytic nanoparticles and/or electroactive particlesdisposed on a surface of the electrically-conductive coating. In thefirst embodiments or any other embodiment, the catalytic nanoparticlescomprise ruthenium (Ru).

In the first embodiments or any other embodiment, a plurality ofnon-wood particles is coupled to one or more surfaces of the flexiblesubstrate. In the first embodiments or any other embodiment, thenon-wood particles coupled to the flexible substrate surface(s) comprisemetallic nanoparticles, metal alloy nanoparticles, semiconductornanoparticles, sulfides, phosphides, borides, or oxides. In the firstembodiments or any other embodiment, the metallic nanoparticles or themetal alloy nanoparticles comprise at least one of Pt, Pd, Au, Ag, Ni,Co, Ru, and Fe. In the first embodiments or any other embodiment, thesemiconductor nanoparticles comprise CuFeSe₂ or any other semiconductor.In the first embodiments or any other embodiment, the sulfides compriseat least one of MoS₂, CoS_(x), and FeS₂, where x is an integer. In thefirst embodiments or any other embodiment, the phosphides comprise atleast one of CoP, NiP₂, and MoP_(x), where x is integer. In the firstembodiments or any other embodiment, the borides comprise at least oneof CoB, MoB, and NiB. In the first embodiments or any other embodiment,the oxides comprise at least one of MnO₂, Fe₂O₃, CoO, and NiO.

In the first embodiments or any other embodiment, theelectrically-conductive coating comprises carbon nanotubes (CNT),graphene, polyaniline, or carbon black.

In the first embodiments or any other embodiment, the flexible substratehas sufficient flexibility so as to be wrapped around and worn on anappendage or another body part of a user and to return to its originalshape when released.

In one or more second embodiments, a method comprises treating a pieceof natural wood with a chemical solution so as to partially removehemicellulose and lignin therein while substantially preserving astructure of cellulose-based lumina, and then drying the piece ofchemically-treated natural wood. The treating and the drying increase aflexibility of said piece.

In the second embodiments or any other embodiment, the drying isperformed in air, such that the cellulose-based lumina crumple in across-sectional view. In the second embodiments or any other embodiment,the drying comprises freeze drying or critical point drying, such thatthe cellulose-based lumina remain open in a cross-sectional view.

In the second embodiments or any other embodiment, the chemical solutionhas a pH greater than 7. In the second embodiments or any otherembodiment, the chemical solution comprises at least one of NaOH,Na₂SO₃, (NH₄)₂SO₃, p-TsOH, NH₃.H₂O, NaOH+Na₂S, Na₂CO₃, NaOH+Na₂SO₃,NaOH+(NH₄)₂SO₃, NH₄OH+(NH₄)₂SO₃, NH₄OH+ Na₂SO₃, NaOH+Na₂CO₃, NaOH+AQ,NaOH/Na₂S+AQ, NaOH+Na₂SO₃+AQ, Na₂SO₃+AQ, NaOH+Na₂S+Na₂S_(n),Na₂SO₃+NaOH+CH₃OH+AQ, C₂H₅OH+NaOH, C₂H₈N₂, C₂H₇NO+NH₃—H₂O, N₂H₄—H₂O, andNaHCO₃, where n in an integer and AQ is Anthraquinone. In the secondembodiments or any other embodiment, the chemical solution comprises amixture of NaOH and Na₂SO₃. In the second embodiments or any otherembodiment, the chemical solution comprises 2.5M of NaOH and 0.4M ofNa₂SO₃. In the second embodiments or any other embodiment, the chemicalsolution is boiling.

In the second embodiments or any other embodiment, the method comprises,before the drying and after the treating, immersing the piece ofchemically-treated natural wood in a solvent to remove remnants of thechemical solution in said piece. In the second embodiments or any otherembodiment, said solvent comprises boiling de-ionized (DI) water.

In the second embodiments or any other embodiment, the treatingcomprises immersing the piece of natural wood in the chemical solutionfor at least ten minutes. In the second embodiments or any otherembodiment, the treating is performed under vacuum, such that thechemical solution penetrates into the lumina of the piece of naturalwood.

In the second embodiments or any other embodiment, at least some of thehemicellulose and lignin are retained by said piece after the treating.In the second embodiments or any other embodiment, between 5% and 95% ofthe lignin has been removed from said piece by the treating. In thesecond embodiments or any other embodiment, said piece retains at least20% of the cellulose after the treating.

In the second embodiments or any other embodiment, a tensile strength ofsaid piece is at least 2 times greater than that before the treating. Inthe second embodiments or any other embodiment, a bending radius of saidpiece is at least 2 times smaller than that before the treating.

In the second embodiments or any other embodiment, the method comprises,after the drying, forming entangled fiber networks within inner volumesof the lumina of said piece. In the second embodiments or any otherembodiment, the treating and drying are such that cellulose nanofiberswithin inner volumes of the lumina are entangled with each other. In thesecond embodiments or any other embodiment, the piece ofchemically-treated natural wood is subjected to the drying after thetreating without any rinsing in between.

In the second embodiments or any other embodiment, said piece is amonolithic block, strip, bar, sheet, or membrane.

In the second embodiments or any other embodiment, cellulose nanofibersof the wood are substantially aligned along a common direction after thetreating, and the treating is such that nanopores are introduced betweenthe aligned cellulose nanofibers. In the second embodiments or any otherembodiment, after the drying, inner volumes of at least some of thecellulose-based lumina of the wood are open or unobstructed.

In the second embodiments or any other embodiment, the piece of naturalwood is hard wood or softwood. In the second embodiments or any otherembodiment, the piece of natural wood is one of basswood, oak, poplar,ash, alder, aspen, balsa wood, beech, birch, cherry, butternut,chestnut, cocobolo, elm, hickory, maple, oak, padauk, plum, walnut,willow, yellow poplar, bald cypress, cedar, cypress, douglas fir, fir,hemlock, larch, pine, redwood, spruce, tamarack, juniper and yew

In the second embodiments or any other embodiment, the method comprises,after the drying, seeding a plurality of non-native cells on the pieceof chemically-treated natural wood. In the second embodiments or anyother embodiment, the method comprises, after the seeding, culturing thenon-native cells to form a cell population, cell culture, or tissue. Inthe second embodiments or any other embodiment, the culturing includestransporting nutrients to the non-native cells and metabolic byproductsfrom the non-native cells via the preserved structure. In the secondembodiments or any other embodiment, the non-native cells are non-woodcells. In the second embodiments or any other embodiment, the non-woodcells are human or animal cells.

In the second embodiments or any other embodiment, the method comprises,after the drying, at least one of bending or folding the piece ofchemically-treated natural wood onto itself, twisting the piece ofchemically-treated natural wood more than 45°, and wrapping the piece ofchemically-treated natural wood around an appendage or another body partof a user. In the second embodiments or any other embodiment, thebending, folding, twisting, or wrapping includes at least twosimultaneous folds or twists. In the second embodiments or any otherembodiment, the method comprises, after the bending, folding, twisting,or wrapping, releasing the piece of chemically-treated natural wood suchthat it returns to its original shape. In the second embodiments or anyother embodiment, the original shape is a substantially flat shape.

In the second embodiments or any other embodiment, the method comprises,after the drying, forming a coating on the piece of chemically-treatednatural wood. In the second embodiments or any other embodiment, thecoating comprises a conductor, a semiconductor, or an insulator. In thesecond embodiments or any other embodiment, the coating comprises atleast one of nanoparticles, nanowires, graphene, graphite, single-walledcarbon nanotubes, double-walled carbon nanotubes, and multi-walledcarbon nanotubes. In the second embodiments or any other embodiment, thecoating comprises at least one of carbon nanotubes (CNT), carbon black,graphite, hard carbon, reduced graphene oxide, graphene, plasmonicmetallic nanoparticles, and semiconductor nanoparticles. In the secondembodiments or any other embodiment, the plasmonic nanoparticlescomprise at least one of Au, Pt, Ag, Pd, and Ru. In the secondembodiments or any other embodiment, the semiconductor nanoparticlescomprise CuFeSe₂ or any other semiconductor.

In the second embodiments or any other embodiment, the coating iscomprised of carbon nanotubes (CNT) and the forming comprises immersingthe piece of chemically-treated natural wood in a CNT solution and then,drying the piece of chemically-treated natural wood. In the secondembodiments or any other embodiment, the method comprises sonicating theCNT solution during the immersing in the CNT solution.

In the second embodiments or any other embodiment, the piece ofchemically-treated natural wood has a first surface perpendicular to adirection in which the lumina extend, and the forming is such that thecoating is formed on the first surface while inner surfaces of thelumina away from the first surface remain uncoated. In the secondembodiments or any other embodiment, the piece of chemically-treatednatural wood has a first surface parallel to a direction in which thelumina extend, and the forming is such that the coating is formed on thefirst surface.

In the second embodiments or any other embodiment, the method comprises,prior to the forming a coating, forming at a surface of said piece aplurality of micro-sheet protrusions extending therefrom, and theforming a coating is such that the coating is formed on the micro-sheetprotrusions. In the second embodiments or any other embodiment, theforming the plurality of micro-sheet protrusions includes cuttingnatural word to form the surface of said piece.

In the second embodiments or any other embodiment, the method comprises,after the forming a coating placing a second surface of the piece ofchemically-treated natural wood, opposite to the first surface, incontact with a fluid source, and exposing the coated first surface to asource of light, such that fluid pumped to the first surface from thesecond surface via capillary action and/or nano-cavitation effects isheated to generate steam or vapor.

In the second embodiments or any other embodiment, the piece ofchemically-treated natural wood has a first surface parallel to adirection in which the lumina extend, and the forming is such that thecoating is formed on at least the first surface and inner surfaces ofthe lumina. In the second embodiments or any other embodiment, the pieceof chemically-treated natural wood has a first surface perpendicular toa direction in which the lumina extend, and the forming is such that thecoating is formed on the first surface and on inner surfaces of thelumina.

In the second embodiments or any other embodiment, the method comprises,after the forming a coating, depositing a plurality of catalyticnanoparticles and/or electroactive particles on a surface of thecoating. In the second embodiments or any other embodiment, thecatalytic nanoparticles comprise ruthenium (Ru).

In the second embodiments or any other embodiment, the method furthercomprises, coupling non-native (e.g., non-wood) particles to one or moresurfaces of said piece of chemically-treated natural wood. In the secondembodiments or any other embodiment, the non-native particles comprisemetallic nanoparticles, metal alloy nanoparticles, sulfides, phosphides,borides, or oxides. In the second embodiments or any other embodiment,the metallic nanoparticles or the metal alloy nanoparticles comprise atleast one of Pt, Pd, Au, Ag, Ni, Co, Ru, and Fe. In the secondembodiments or any other embodiment, the sulfides comprise at least oneof MoS₂, CoS_(x), and FeS₂, where x is an integer. In the secondembodiments or any other embodiment, the phosphides comprise at leastone of CoP, NiP₂, and MoP_(x), where x is an integer. In the secondembodiments or any other embodiment, the borides comprise at least oneof CoB, MoB, and NiB. In the second embodiments or any other embodiment,the oxides comprise at least one of MnO₂, Fe₂O₃, CoO, and NiO.

In the second embodiments or any other embodiment, the method comprisescoupling to one or more surfaces of the piece of chemically-treatednatural wood at least one of nanoparticles, nanowires, graphene,graphite, single-walled carbon nanotubes, double-walled carbonnanotubes, and multi-walled carbon nanotubes. In the second embodimentsor any other embodiment, the method comprises intercalating in one ormore cellulose walls of the lumina of the piece of chemically-treatednatural wood at least one of nanoparticles, nanowires, graphene,graphite, single-walled carbon nanotubes, double-walled carbonnanotubes, and multi-walled carbon nanotubes.

In the second embodiments or any other embodiment, the method comprises,filling the lumina of the chemically-treated natural wood with aflexible polymer. In the second embodiments or any other embodiment, theflexible polymer comprises a silicone polymer. In the second embodimentsor any other embodiment, the method comprises, prior to the filling witha flexible polymer, forming a coating on the piece of chemically-treatednatural wood. In the second embodiments or any other embodiment, thecoating comprises a conductor, a semiconductor, or an insulator.

In the second embodiments or any other embodiment, the method comprises,after the depositing, disposing a separator membrane between an anodeand a second surface of the piece of chemically-treated natural wood,the second surface being opposite to the first surface, and filling atleast the piece of chemically-treated natural wood with an electrolytesolution. In the second embodiments or any other embodiment, the methodcomprises applying a voltage between the piece of chemically-treatednatural wood and the anode so as to store charge therein.

In one or more third embodiments, a structure is formed by treatingnatural wood with a chemical solution that partially removeshemicellulose and lignin therein while substantially preserving astructure of cellulose-based lumina, and the treated piece of naturalwood has a bending radius that is at least 2 times smaller than that ofthe natural wood before treatment.

In the third embodiments or any other embodiment, the treated naturalwood has sufficient flexibility so as to return to its original shapeafter being folded onto itself or being twisted more than 45°.

In the third embodiments or any other embodiment, the treated naturalwood has a continuous first surface perpendicular to a direction inwhich the lumina extend, and a continuous second surface opposite to thefirst surface.

In the third embodiments or any other embodiment, between 5% and 95% ofthe lignin in the natural wood has been removed by the treating with thechemical solution.

In the third embodiments or any other embodiment, the treating comprisesimmersing the natural wood in the chemical solution for a first timeand, after the immersing, drying the natural wood. In the thirdembodiments or any other embodiment, the chemical solution has a pHgreater than 7 and comprises at least one of NaOH, Na₂SO₃, (NH₄)₂SO₃,p-TsOH, NH₃.H₂O, NaOH+Na₂S, Na₂CO₃, NaOH+Na₂SO₃, NaOH+(NH₄)₂SO₃,NH₄OH+(NH₄)₂SO₃, NH₄OH+Na₂SO₃, NaOH+Na₂CO₃, NaOH+AQ, NaOH/Na₂S+AQ,NaOH+Na₂SO₃+AQ, Na₂SO₃+AQ, NaOH+Na₂S+Na₂S_(n), Na₂SO₃+NaOH+CH₃OH+AQ,C₂H₅OH+NaOH, C₂H₈N₂, C₂H₇NO+NH₃—H₂O, N₂H₄—H₂O, and NaHCO₃, where n in aninteger and AQ is Anthraquinone. In the third embodiments or any otherembodiment, the chemical solution comprises a mixture of NaOH andNa₂SO₃, and the first time is a least ten minutes.

In one or more fourth embodiments, a hybrid structure comprises thestructure of any of the first and third embodiments, or a structureformed by the method of any of the second embodiments, and at least oneof nanoparticles, nanowires, graphene, graphite, single-walled carbonnanotubes, double-walled carbon nanotubes, and multi-walled carbonnanotubes coupled to a surface of said structure.

In one or more fifth embodiments, a biological tissue scaffold comprisesthe hybrid structure of any of the fourth embodiments, the structure ofany of the first and third embodiments, or a structure formed by themethod of any of the second embodiments.

In one or more sixth embodiments, a structural or building materialcomprises the hybrid structure of any of the fourth embodiments, thestructure of any of the first and third embodiments, or a structureformed by the method of any of the second embodiments.

In one or more seventh embodiments, a steam or vapor generation systemcomprises the hybrid structure of any of the fourth embodiments, thestructure of any of the first and third embodiments, or a structureformed by the method of any of the second embodiments.

In one or more eight embodiments, a distillation system comprises thesteam or vapor generation system of any of the seventh embodiments.

In one or more ninth embodiments, battery comprises the hybrid structureof any of the fourth embodiments, the structure of any of the first andthird embodiments, or a structure formed by the method of any of thesecond embodiments.

In one or more tenth embodiments, an electronic device comprises thebattery of any of the ninth embodiments.

In one or more eleventh embodiments, a conductor or electronic devicecomprises the hybrid structure of any of the fourth embodiments, thestructure of any of the first and third embodiments, or a structureformed by the method of any of the second embodiments.

In one or more twelfth embodiments, a wearable device comprises thehybrid structure of any of the fourth embodiments, the structure of anyof the first and third embodiments, or a structure formed by the methodof any of the second embodiments, wherein the structure is constructedto be wrapped around and worn on an appendage or other body part of auser.

In one or more thirteenth embodiments, a shape memory device comprisesthe hybrid structure of any of the fourth embodiments, the structure ofany of the first and third embodiments, or a structure formed by themethod of any of the second embodiments, wherein the structure isconstructed to return to its original shape after release of a forcedeforming the structure.

In one or more fourteenth embodiments, a hybrid structure comprises aflexible substrate of natural wood that has been chemically modified topartially remove hemicellulose and lignin therein while substantiallypreserving a structure of cellulose-based lumina, and a non-nativematerial coupled to surfaces of the lumina.

In the fourteenth embodiments or any other embodiment, the non-nativematerial is a non-wood material. In the fourteenth embodiments or anyother embodiment, the non-wood material comprises at least one ofnanoparticles, nanowires, and a coating. In the fourteenth embodimentsor any other embodiment, the non-wood material is conducting,semiconducting, or insulating. In the fourteenth embodiments or anyother embodiment, the coating comprises at least one of graphene,graphite, single-walled carbon nanotubes, double-walled carbonnanotubes, multi-walled carbon nanotubes, nanoparticles, sulfides,phosphides, borides, and oxides. In the fourteenth embodiments or anyother embodiment, the nanoparticles comprise at least one of Pt, Pd, Au,Ag, Ni, Co, Ru, Fe, and CuFeSe₂. In the fourteenth embodiments or anyother embodiment, the sulfides comprise at least one of MoS₂, CoS_(x),and FeS₂, where x is an integer. In the fourteenth embodiments or anyother embodiment, the phosphides comprise at least one of CoP, NiP₂, andMoP_(x), where x is an integer. In the fourteenth embodiments or anyother embodiment, the borides comprise at least one of CoB, MoB, andNiB. In the fourteenth embodiments or any other embodiment, the oxidescomprise at least one of MnO₂, Fe₂O₃, CoO, and NiO.

In the fourteenth embodiments or any other embodiment, the non-woodmaterial comprises a flexible polymer at least partially filling thelumina. In the fourteenth embodiments or any other embodiment, theflexible polymer comprises a silicone polymer.

In the fourteenth embodiments or any other embodiment, the hybridstructure includes conductive portions and is configured as anelectronic device.

In this application, unless specifically stated otherwise, the use ofthe singular includes the plural, and the separate use of “or” and “and”includes the other, i.e., “and/or.” Furthermore, use of the terms“including” or “having,” as well as other forms such as “includes,”“included,” “has,” or “had,” are intended to have the same effect as“comprising” and thus should not be understood as limiting.

Any range described herein will be understood to include the endpointsand all values between the endpoints. Whenever “substantially,”“approximately,” “essentially,” “near,” or similar language is used incombination with a specific value, variations up to and including 10% ofthat value are intended, unless explicitly stated otherwise.

The foregoing descriptions apply, in some cases, to examples generatedin a laboratory, but these examples can be extended to productiontechniques. Thus, where quantities and techniques apply to thelaboratory examples, they should not be understood as limiting.

It is thus apparent that there is provided, in accordance with thepresent disclosure, flexible wood structures and devices, and methodsfor fabricating and use thereof. Many alternatives, modifications, andvariations are enabled by the present disclosure. While specificexamples have been shown and described in detail to illustrate theapplication of the principles of the present invention, it will beunderstood that the invention may be embodied otherwise withoutdeparting from such principles. For example, disclosed features may becombined, rearranged, omitted, etc. to produce additional embodiments,while certain disclosed features may sometimes be used to advantagewithout a corresponding use of other features. Accordingly, Applicantintends to embrace all such alternative, modifications, equivalents, andvariations that are within the spirit and scope of the presentinvention.

The invention claimed is:
 1. A structure comprising: a flexiblesubstrate of natural wood that has been chemically modified to partiallyremove hemicellulose and lignin therein while retaining cellulose-basedlumina defined by wood cell walls formed by aligned cellulosenanofibers; and a coating on one or more external surfaces of theflexible substrate, wherein inner volumes of the retainedcellulose-based lumina are open or unobstructed.
 2. The structure ofclaim 1, wherein between 5% and 95% of the lignin has been removed bythe chemical modification.
 3. The structure of claim 1, wherein thecoating comprises nanoparticles, nanowires, graphene, graphite,single-walled carbon nanotubes, double-walled carbon nanotubes,multi-walled carbon nanotubes, or any combination of the foregoing. 4.The structure of claim 1, wherein the coating is a light-absorbingcoating comprising carbon nanotubes (CNT), carbon black, graphite, hardcarbon, reduced graphene oxide, graphene, plasmonic metallicnanoparticles, semiconductor nanoparticles, or any combination of theforegoing.
 5. The structure of claim 4, further comprising: a capturehood disposed over the flexible substrate so as to capture vaporgenerated by the flexible substrate; and a condenser coupled to thecapture hood so as to receive the captured vapor, the condenser beingconfigured to convert the captured vapor to a liquid.
 6. The structureof claim 1, wherein: a first one of the one or more external surfaceshas a plurality of micro-sheet protrusions extending therefrom; and thecoating comprises an electrically-conductive portion disposed on thefirst one of the one or more external surfaces having the plurality ofmicro-sheet protrusions and on inner surfaces forming thecellulose-based lumina within the flexible substrate.
 7. The structureof claim 1, further comprising: a plurality of catalytic nanoparticlesand/or electroactive particles disposed on a surface of the coating,wherein the coating is electrically-conductive.
 8. The structure ofclaim 7, wherein the plurality of catalytic nanoparticles comprisesruthenium (Ru).
 9. The structure of claim 1, further comprising: aplurality of nanoparticles disposed on a surface of the coating, whereinthe plurality of nanoparticles comprises Pt, Pd, Au, Ag, Ni, Co, Ru, Fe,CuFeSe₂ or any combination of the foregoing.
 10. The structure of claim1, wherein the aligned cellulose nanofibers within the flexiblesubstrate are aligned along a common direction.
 11. The structure ofclaim 1, wherein the wood cell walls of the retained cellulose-basedlumina have nanopores between the aligned cellulose nanofibers.
 12. Thestructure of claim 1, wherein: a first one of the one or more externalsurfaces has a plurality of micro-sheet protrusions extending therefrom;and the coating comprises a portion disposed on the first one of the oneor more external surfaces having the plurality of micro-sheetprotrusions but not on inner surfaces of the retained cellulose-basedlumina away from the first one of the one or more external surfaces. 13.A structure comprising: a flexible substrate of natural wood that hasbeen chemically modified to partially remove hemicellulose and lignintherein while retaining cellulose-based lumina defined by wood cellwalls formed by aligned cellulose nanofibers; and a plurality ofnon-wood cells attached to cell wall surfaces defining thecellulose-based lumina in the flexible substrate, wherein the flexiblesubstrate acts as a tissue scaffold for the plurality of non-wood cells.14. The structure of claim 13, wherein the plurality of non-wood cellsforms a tissue on the flexible substrate.
 15. A structure comprising: aflexible substrate of natural wood that has been chemically modified topartially remove hemicellulose and lignin therein while retainingcellulose-based lumina defined by wood cell walls formed by alignedcellulose nanofibers; an electrically-conductive coating on the flexiblesubstrate; an anode; an electrolyte solution filling at least theflexible substrate; and a separator membrane disposed between theflexible substrate and the anode, wherein the flexible substrate definesfirst pathways through the wood cell walls for transport of ions, secondpathways through inner volumes of the cellulose-based lumina fortransport of a gas, and third pathways via the electrically-conductivecoating for transport of electrons, the flexible substrate acts acurrent collector-free cathode, and the structure is configured as abattery.
 16. The structure of claim 15, wherein the ions transported bythe first pathways are lithium ions (Li⁺), the gas transported by thesecond pathways is oxygen gas (O₂), and the structure is configured as aLi—O₂ battery.